Catalyzed chemical reactions are widely used and are commercially very important. As a result, the development of new catalysts and catalyzed processes has been the object of a significant amount of technical development. The development of new catalysts and catalyzed reactions has been hampered by the difficulty encountered in obtaining basic information about the physical and chemical processes involved in catalytic activity and catalytic reactions, such as reaction intermediates, reaction mechanisms, adsorption and desorption of reactants and products in catalytic reactions, oxidation and reduction of catalysts, catalyst poisons, the concentration of reactants on a catalyst surface, and others.
Classically, this kind of basic information about the chemical and physical processes of catalysis has been deduced primarily from analysis of the final products of the reaction. Conclusions have been based on final products because of the difficulty in isolating and analyzing reaction intermediates, many of which are highly fragile and reactive species. Being able to determine directly the identity of these intermediates and to follow their production and consumption during the reaction would increase the understanding of catalysis and would facilitate the development of catalysts and catalytic processes.
One method that has been used to study the interaction of catalytic surfaces with reactant molecules is called molecular beam mass spectrometry. In this technique, a stream of molecules of reactant gas (a molecular beam) is directed at a target of the catalytic material, with the target oriented at an angle to the molecular beam. The molecules of the reactant gas strike the target, some of them react to form products and - intermediates, and they rebound off the target in the direction of an aperture. A portion of the rebounding molecules pass through the aperture into the ionization chamber of a mass spectrometer, which analyzes the mixture for reactants, intermediates, and products. A variation on this molecular beam technique is called modulated molecular beam mass spectrometry, in which the initial molecular beam of reactant gas is modulated, such as with a rotating "chopper", to produce a series of pulses of the reactant gas. The result is that a series of pulses of gas enter the mass spectrometer for analysis.
In these molecular beam techniques, the entire assembly is enclosed and is operated in a vacuum. The vacuum is necessary to achieve the molecular flow to form the molecular beam, and is necessary for operation of the mass spectrometer.
The vacuum required, along with the fact that the molecules strike the catalyst target and rebound to the detector combine to make the number of reaction opportunities for each molecule of reactant very small. It has been estimated that the number of collisions between a given molecule of reactant gas and the target catalyst would be 10 or less, and that the number of collisions between a given molecule of reactant gas and other gas molecules would also be 10 or less. This means that these molecular beam techniques are practical only for highly reactive systems, in which sufficient reaction occurs in the small number of reaction opportunities to produce detectable amounts of products and intermediates. Most commercially important catalyzed reaction systems are not reactive enough for use with molecular beam techniques. The catalyst suitable for use with molecular beam techniques must be made into a target with a surface regular enough so that the direction of rebound of the reactant gas molecules can be directed toward the mass spectrometer. Not all catalysts can be formed into such a target.
Conventional techniques have been adapted to try to isolate and analyze for reaction intermediates. One common technique involves a reactor containing a catalyst, through which an inert carrier gas flows continuously. A pulse of reactant gas is injected into the carrier gas and is carried through the catalyst. As the product gas exits the reactor, samples are taken and analyzed. This type of system is normally operated at or near atmospheric pressure. The number of collisions between an average molecule of reactant gas and the catalyst is very high, and has been estimated to be far greater than 10.sup.6. Similarly, the number of collisions between an average molecule of reactant gas and other gas molecules has been estimated to be far greater than 10.sup.6. Due to the large number of reaction opportunities, the number of fragile and highly reactive intermediates that emerge from the catalyst is very small, and is usually too small to be detected.
The method and apparatus of this invention overcome some of the problems associated with prior art techniques to study catalysis. This invention preserves and detects fragile and highly reactive reaction intermediates of catalyzed chemical reactions, and preserves the time sequence of reactant/intermediate/product species evolved in a catalyzed chemical reaction. Practice of the novel process of the invention further requires provision of systems for delivery, temperature control and mixing of inlet gas, as well as reaction systems which can be operated under temperature control to produce a pulse of product gases that can be analyzed to provide meaningful information on the kinetics, reaction equilibria, and adsorption/desorption phenomena involved in the catalytic reaction.
A particular need exists for an apparatus operating under substantial vacuum which allows effective evaluation of catalytic reaction systems in which one or more of the feed materials has a relatively low vapor pressure at ambient temperature. For example, in studying the oxidation of butane to maleic anhydride, it may be desirable to feed maleic anhydride to the catalyst bed in order to investigate its decomposition over the catalyst. However, at room temperatures maleic anhydride has a vapor pressure of less than one torr, which is insufficient for such experiments. To circumvent and solve this problem, the vapor pressure of maleic anhydride or other low volatility feed materials may be raised by heating the maleic anhydride to elevated temperatures. This, however, poses special problems since not only the sample container must be heated but also all of the sample feed lines and the contacted valve parts must be heated. If they are not heated, the vapor will condense on the cooler surfaces.